Section on Cognitive Neurophysiology and Imaging, Laboratory of Neuropsychology, National Institute of Mental Health (NIMH), National Institutes of Health (NIH), Department of Health and Human Services

Michael Schmid

Section on Cognitive Neurophysiology and Imaging, Laboratory of Neuropsychology, National Institute of Mental Health (NIMH), National Institutes of Health (NIH), Department of Health and Human Services

Andrew Peters

Section on Cognitive Neurophysiology and Imaging, Laboratory of Neuropsychology, National Institute of Mental Health (NIMH), National Institutes of Health (NIH), Department of Health and Human Services

Richard Saunders

Section on Cognitive Neurophysiology and Imaging, Laboratory of Neuropsychology, National Institute of Mental Health (NIMH), National Institutes of Health (NIH), Department of Health and Human Services

David Leopold

Section on Cognitive Neurophysiology and Imaging, Laboratory of Neuropsychology, National Institute of Mental Health (NIMH), National Institutes of Health (NIH), Department of Health and Human Services

Alexander Maier

Section on Cognitive Neurophysiology and Imaging, Laboratory of Neuropsychology, National Institute of Mental Health (NIMH), National Institutes of Health (NIH), Department of Health and Human Services

Illusory figures are a powerful demonstration of the visual systems ability to infer object boundaries and surfaces under conditions of fragmented sensory input. Neural correlates of illusory figures have been observed in a wide range of brain areas. Recordings in monkeys revealed that illusory figures evoke spiking responses from neurons in visual areas as early as V1 and V2 and as late as the inferotemporal cortex. Similarly, neuroimaging studies in humans identified responses to illusory figures throughout visual cortex. One area of particular interest is V4, as the receptive fields of its neurons are large enough to cover the separate stimulus elements and sensitive enough to distinguish between local features such as orientation, curvature, and colinearity. In order to investigate the role of mid-level visual area V4 in visual surface completion, we used extracellular multi-electrode arrays to measure spiking responses and low-frequency activity to two types of visual stimuli: Kanizsa patterns that induce the perception of an illusory surface and physically similar control stimuli that do not. Neurons in V4 exhibited stronger and sometimes rhythmic spiking responses for the illusion-promoting configurations compared to controls. Moreover, this elevated response depended on the precise alignment of the neurons peak visual field sensitivity ("RF-focus") with the illusory surface itself. Neurons whose RF-focus over adjacent inducing elements did not show response enhancement to the illusion compared to the control stimuli. This spatial sensitivity suggests that, despite having large receptive fields, V4 neurons are able to draw upon spatially specific input, and that the observed response enhancement associated with the illusory surface may be computed before or within area V4. These finding will be discussed in relation to V4s functional domains as well as the putative role of rhythmic neural activity for the integration of feedforward and feedback signals in early visual cortex.